The invention relates generally to photocopiers and in particular to a method and apparatus for stripping sheets from a fragile photosensitive surface after transfer of a developed image from the surface to a copy sheet.
A photocopier employs a moving photosensitive surface which is electrically charged and exposed to an energy image, most often a light energy pattern generated by scanning an original document. The energy is focused on the charged surface to form a latent electrostatic image thereon. The latent image is developed, and the developed image is transferred to a copy sheet of transfer material brought into intimate contact with the surface portion bearing the developed image. The sheet is then stripped from the photosensitive surface and transported away from the surface along an exit path. A fixing or fusing station is typically provided along the exit path.
The copy sheet, when it is brought into contact with the photosensitive surface, ordinarily adheres tightly to it due to the electrostatic fields created during the photocopying and transfer processes and, in the case of liquid developer copiers, also because of the surface tension created by the liquid developer. The sheet must be stripped from the surface with a minimum of disturbance to the generally fragile developed image and with minimum or no damage to the fragile photosensitive surface. One commercially available dry copier directs a short-duration stream of air between the copy sheet and the photosensitive surface to separate the leading edge of the copy sheet so that it can be either later gripped or allowed to fall away from the photosensitive surface. The copy sheet is then conveyed to a fixing or fusion station. The air puffing method has the disadvantage of potentially disturbing the toner particles adhering to the copy sheet and of being very sensitive to the system operating parameters. Also it does not operate satisfactorily with liquid developer systems.
There also exist many different apparatus wherein a mechanical member makes actual contact with the fragile photosensitive surface. The member can be in constant contact with the surface, or to reduce the likelihood of damage to the surface, can be brought into timed engagement with the photosensitive surface. In either instance, the stripped paper is allowed to "fall" onto a conveyor system for removal to an exit station or is directed to rollers spaced some distance away from the surface. This apparatus has the disadvantage of potentially marring the image and, where movement is timed to the copy sheet, of requiring the device to be properly timed and precisely positioned so that it does not mar the photosensitive surface or the fragile image and so that sheet pick-off is reliably accomplished.
Another method, particularly useful in connection with liquid developer photocopiers and described more fully in U.S. Pat. No. 4,000,942, provides for a stripping assembly having an elongated blade member in contact with a marginal edge of the photosensitive surface. When the copy sheet is fed toward the photosensitive surface, a marginal edge of the copy sheet slides along the stripping blade member. The member directs a forward portion of a leading edge of the copy sheet to engagement with a roller nip. A disadvantage of this approach is that an entire marginal edge of the copy sheet is prevented from contacting the photosensitive surface and is therefore unavailable for copying. The pressure of the member against the photosensitive surface also can wear or damage the surface.
It is therefore a principal object of this invention to minimize interference with the photocopying process and reduce potential smudging or smearing of the copy sheet image. It is another important object of the invention to maximize the amount of copy sheet surface available for copying. A further object of the invention is a sheet stripping apparatus which is reliable, durable, simple in construction, simple to install, and substantially maintenance free. A yet further object of the invention is a sheet stripping apparatus and method which can be particularly employed in liquid developer copiers.